Trim and Optimal RPM for 7.4L
 

My next Mariah newbie question revolves around trim and optimal engine RPM. My Z280 has the non-Magnum version of the 7.4L 454 engine. My service manager said the optimum for that engine is 3,200 rpm. At that rpm adjusting the trim tabs and power trim on the drive I maxed out at about 22 mph according to the GPS. When I was running a week ago at around 4,000 rpm adjusting both trims the engine seemed to labor less and I maxed out at around 33 mph. My question to you veterans is what is the optimum rpm range for best gas mileage and efficiency.

Re: Trim and Optimal RPM for 7.4L
 

I have the same engine in a smaller 258 Shabah. I would agree that 3200 should be very close to optimum. However that is about 34-35 for my boat. What is WOT? You might be a little under propped?

Re: Trim and Optimal RPM for 7.4L
 

I agree, I think 3200RPM *may* be ideal for my boat, but remember each hull is different, and his Z280 weighs a good 3k more pounds than ours. The only real way to determine the "sweet spot" is by installing a fuel flow meter and just experimenting around with the tabs.

Re: Trim and Optimal RPM for 7.4L
 

Chris,
Not underproped but rather overweight. My Z280 is about 7,200 vs. your 4,200 lbs. But thanks for corroborating the RPM optimum.
Bob

Re: Trim and Optimal RPM for 7.4L
 

7200# + 1000# fuel if tank is full, plus cargo(you)
I had a Z252 with the 350MPI cruised beautifully at 30mph while reving about 2800.

I now have a Z280 as well(but with the 502MPI), only had the test drive last fall, but did manage to top 50MPH
after she was trimed up.
Look forward to more info in the spring as we play with them.

Larry